The neutron,gamma-ray,X-ray spectrometer (NGXS): A compact instrument for making combined measurements of neutrons,gamma-rays,and X-rays

The Neutron, Gamma ray, and X-ray Spectrometer (NGXS) is a compact instrument designed to detect neutrons, gamma-rays, and hard X-rays. The original goal of NGXS was to detect and characterize neutrons, gamma-rays, and X-rays from the Sun as part of the Solar Probe Plus mission in order to provide direct insight into particle acceleration, magnetic reconnection, and cross-field transport processes that take place near the Sun. Based on high-energy neutron detections from prompt solar flares, it is estimated that the NGXS would detect neutrons from 15 to 24 impulsive flares. The NGXS sensitivity to 2.2 MeV gamma rays would enable a detection of ∼50–60 impulsive flares. The NGXS is estimated to measure ∼120 counts/s for a GOES C1-type flare at 0.1 AU, which allows for a large dynamic range to detect both small and large flares.     

Interaction of mental and orthostatic stressors

We assessed hemodynamic responses induced by orthostatic and mental stressors, using passive head up tilt (HUT) and mental arithmetic (MA), respectively. The 15 healthy males underwent three protocols: (1) HUT alone, (2) MA in supine position and (3) MA+HUT, with sessions randomized and ≥2 weeks apart. In relation to baseline, HUT increased heart rate (HR) (+20.4±7.1 bpm; p<0.001), mean blood pressure (MBP) (+4.7±11.3 mmHg; p<0.05), diastolic blood pressure (DBP) (+6.1±11.6 mmHg; p<0.05) and total peripheral resistance (TPR) (+155±232 dyne*s/cm⁵; p<0.001) but decreased stroke volume (SV) (?33.1±13.4 ml; p<0.001) and cardiac output (CO) (?0.6±1.0 l/min; p<0.01). MA increased HR (+8.0±6.0 bpm; p<0.001), systolic blood pressure (SBP) (+9.0±7.7 mmHg; p<0.001), MBP (+10.0±6.5 mmHg; p<0.001), DBP (+9.5±7.2 mmHg; p<0.001) and CO (+0.6±0.8 l/min; p<0.01). MA+HUT increased HR (+28.8±8.4 bpm; p<0.001), SBP (+4.6±14.3 mmHg; p<0.05), MBP (+11.2±11.6 mmHg; p<0.001), DBP (+13.5±10.1 mmHg; p<0.001) and TPR (+160±199 dyne*s/cm⁵; p<0.001) but SV (?34.5±14.6 ml; p<0.001) decreased. Mental challenge during orthostatic challenge elicited greater increases in heart rate, despite similar reductions in stroke volume such as those during orthostatic stress alone. Overall, cardiac output decreases were less with combinations of mental and orthostatic challenges in comparison to orthostasis alone. This would suggest that carefully chosen mental stressors might affect orthostatic responses of people on standing up. Therefore, additional mental loading could be a useful countermeasure to alleviate the orthostatic responses of persons, particularly in those with histories of dizziness on standing up or on return to earth from the spaceflight environment of microgravity.     

Radiation engineering analysis of shielding materials to assess their ability to protect astronauts in deep space from energetic particle radiation

An analysis is performed on four typical materials (aluminum, liquid hydrogen, polyethylene, and water) to assess their impact on the length of time an astronaut can stay in deep space and not exceed a design basis radiation exposure of 150 mSv. A large number of heavy lift launches of pure shielding mass are needed to enable long duration, deep space missions to keep astronauts at or below the exposure value with shielding provided by the vehicle. Therefore, vehicle mass using the assumptions in the paper cannot be the sole shielding mechanism for long duration, deep space missions. As an example, to enable the Mars Design Reference Mission 5.0 with a 400 day transit to and from Mars, not including the 500 day stay on the surface, a minimum of 24 heavy lift launches of polyethylene at 89,375 lbm (40.54 tonnes) each are needed for the 1977 galactic cosmic ray environment. With the assumptions used in this paper, a single heavy lift launch of water or polyethylene can protect astronauts for a 130 day mission before exceeding the exposure value. Liquid hydrogen can only protect the astronauts for 160 days. Even a single launch of pure shielding material cannot protect an astronaut in deep space for more than 180 days using the assumptions adopted in the analysis. It is shown that liquid hydrogen is not the best shielding material for the same mass as polyethylene for missions that last longer than 225 days.     

Right ventricular tissue Doppler assessment in space during circulating volume modification using the Braslet device

Introduction: This joint US–Russian work aims to establish a methodology for assessing cardiac function in microgravity in association with manipulation of central circulating volume. Russian Braslet-M (Braslet) occlusion cuffs were used to temporarily increase the volume of blood in the lower extremities, effectively reducing the volume in central circulation. The methodology was tested at the International Space Station (ISS) to assess the volume status of crewmembers by evaluating the responses to application and release of the cuffs, as well as to modified Valsalva and Mueller maneuvers. This case study examines the use of tissue Doppler (TD) of the right ventricular (RV) free wall. Results: Baseline TD of the RV free wall without Braslet showed early diastolic E′ (16 cm/s), late diastolic A′ (14 cm/s), and systolic S′ (12 cm/s) velocities comparable with those in normal subjects on Earth. Braslet application caused 50% decrease of E′ (8 cm/s), 45% increase of A′, and no change to S′. Approximately 8 beats after the Braslet release, TD showed E′ of 8 cm/s, A′ of 12 cm/s, and S′ of 13 cm/s. At this point after release, E′ did not recover to baseline values while l A′ and S′ did recover. The pre-systolic cross-sectional area of the internal jugular vein without Braslet was 1.07 cm², and 1.13 cm² 10 min after the Braslet was applied. The respective cross-sectional areas of the femoral vein were 0.50 and 0.54 cm². The RV myocardial performance Tei index was calculated by dividing the sum of the isovolumic contraction time and isovolumic relaxation time by the ejection time ((IVCT+IVRT)/ET); baseline and Braslet-on values for Tei index were 0.25 and 0.22, respectively. Braslet Tei indices are within normal ranges found in healthy terrestrial subjects and temporarily become greater than 0.4 during the dynamic Braslet release portion of the study. Conclusions: TD modality was successfully implemented in space flight for the first time. TD of RV revealed that the Braslet influenced cardiac preload and that fluid was sequestered in the lower extremity interstitial and vascular space after only 10 min of application. This report demonstrates that Braslet application has an effect on RV physiology in long-duration space flight based on TD, and that this effect is in part due to venous hemodynamics.     

Launch and early operations of Herschel and Planck

On 14 May 2009 the European Space Agency launched 2 space observatories: Herschel (with a 3.5 m mirror it is the largest space telescope ever) will collect long-wavelength infrared radiation and will be the only space observatory to cover the spectral range from far-infrared to sub-millimetre wavelengths, and Planck will look back at the dawn of time, close to the Big Bang, and will examine the Cosmic Microwave Background (CMB) radiation to a sensitivity, angular resolution and frequency range never achieved before. This paper will present the Flight Dynamics, mission analysis challenges and flight results from the first 3 months of these missions.Both satellites were launched on the same Ariane 5 and travelled to the L2 Lagrange point of the sun–earth system 1.5 million km from the earth in the opposite direction of the sun. There they were injected to a quasi-halo orbit (Herschel) with the dimension of typically 750,000 km×450,000 km, and a Lissajous orbit (Planck) of 300,000 km×300,000 km.In order to reach these Lissajous orbits it is mandatory to perform large trajectory correction manoeuvres during the first days of the mission. Herschel had its main manoeuvres on the first day. Planck had to be navigated on the first day and by a mid-course correction manoeuvre, the L2 orbit insertion manoeuvre was planned on day 50. If these slots were missed, fuel penalties would rapidly increase.This posed a heavy load on the operations teams because both spacecrafts have to be thoroughly checked out and put into the correct modes of their attitude control systems during the first hours after launch.The sequence of events will be presented and explained and the orbit determination results as well as the manoeuvre planning will be emphasised.     

Endurance testing of a pulsed plasma thruster for nanosatellites

The mission complexity of Nanosatellites has increased tremendously in recent years, but their mission range is limited due to the lack of an active orbit control or ∆v capability. Pulsed Plasma Thrusters (PPT), featuring structural simplicity and very low power consumption are a prime candidate for such applications. However, the required miniaturization of standard PPTs and the adaption to the low power consumption is not straightforward. Most investigated systems have failed to show the required lifetime. The present coaxial design has shown a lifetime of up to 1 million discharges at discharge energies of 1.8 J in previous studies. The present paper focuses on performance characterizations of this design. For this purpose direct thrust measurements with a µN thrust balance were conducted. Thrust measurements in conjunction with mass bit determination allowed a comprehensive assessment. Based on those measurements the present µPPT has a total impulses capability of approximately I≈1.7 Ns, an average mass bit of 0.37 µg s⁻¹ and an average specific impulse of I_sp≈904 s. All tests have shown very good EM compatibility of the PPT with the electronics of the flight-like printed circuit board. Consequently, a complete µPPT unit can provide a ∆v change of 5.1 m/s or 2.6 m/s to a standard 1-unit or 2-unit CubeSat respectively.     

Dose estimates in a lunar shelter with regolith shielding

Beyond the Earth's atmosphere, galactic cosmic radiation (GCR) and solar energetic particles (SEPs) are a significant hazard to both manned and robotic missions. For long human missions on the lunar surface (months to a year) a radiation shelter is needed for dose mitigation and emergency protection in case of solar events. This paper investigates the interaction of source protons of solar events like those of February 1956 that emitted many fewer particles with energies up to 1000 MeV and of the October 1989 event of lower protons energy but higher fluence, with the lunar regolith and aluminum shielding of a lunar shelter. The shelter is 5 m in diameter and has a footprint of 5×8 m and a 10 cm thick aluminum support structure, however, actual thickness could be much smaller (～1–2 cm) depending on the weight of the regolith shielding piled on top. The regolith is shown to be slightly more effective than aluminum. Thus, the current results are still applicable for a thinner aluminum structure and increased equivalent (or same mass) thickness of the regolith. The shielding thicknesses to reduce the dose solely due to solar protons in the lunar shelter below those recommended by NASA to astronauts for 30 day-operation in space (250 mSv) and for radiation workers (50 mSv) are determined and compared. The relative attenuation of incident solar protons with regolith shielding and the dose estimates inside the shelter are calculated for center seeking, planar, and isotropic incidence of the source protons. With the center seeking incidence, the dose estimates are the highest, followed by those with isotropic incidence, and the lowest are those with the planar incidence.     

Parabolic maneuvers of the Swiss Air Force fighter jet F-5E as a research platform for cell culture experiments in microgravity

Long-term sensitivity of human cells to reduced gravity has been supposed since the first Apollo missions and was demonstrated during several space missions in the past. However, little information is available on primary and rapid gravi-responsive elements in mammalian cells. In search of rapid-responsive molecular alterations in mammalian cells, short-term microgravity provided by parabolic flight maneuvers is an ideal way to elucidate such initial and primary effects. Modern biomedical research at the cellular and molecular level requires frequent repetition of experiments that are usually performed in sequences of experiments and analyses. Therefore, a research platform on Earth providing frequent, easy and repeated access to real microgravity for cell culture experiments is strongly desired. For this reason, we developed a research platform onboard the military fighter jet aircraft Northrop F-5E “Tiger II”. The experimental system consists of a programmable and automatically operated system composed of six individual experiment modules, placed in the front compartment, which work completely independent of the aircraft systems. Signal transduction pathways in cultured human cells can be investigated after the addition of an activator solution at the onset of microgravity and a fixative or lysis buffer after termination of microgravity. Before the beginning of a regular military training flight, a parabolic maneuver was executed. After a 1 g control phase, the parabolic maneuver starts at 13,000 ft and at Mach 0.99 airspeed, where a 22 s climb with an acceleration of 2.5g is initiated, following a free-fall ballistic Keplerian trajectory lasting 45 s with an apogee of 27,000 ft at Mach 0.4 airspeed. Temperature, pressure and acceleration are monitored constantly during the entire flight. Cells and activator solutions are kept at 37 °C during the entire experiment until the fixative has been added. The parabolic flight profile provides up to 45 s of microgravity at a quality of 0.05g in all axes. Access time is 30 min before take-off; retrieval time is 30 min after landing. We conclude that using military fighter jets for microgravity research is a valuable tool for frequent and repeated cell culture experiments and therefore for state-of-the art method of biomedical research.     

Investigation of nuclear electric powered interstellar precursor missions

Nuclear Electric Propulsion (NEP) is a technology conceptually proposed since the 1940s by E. Stuhlinger in Germany. The JIMO mission originally planned by NASA in the early 2000s produced at least two designs of ion thrusters fed by a 20–30 kW nuclear powerplant. When compared to conventional (chemical) propulsion, the major advantage of NEP in the JIMO context was recognized to be the much higher I_sp (lab-tested at up to 15,000 s) and the capability for sustained power generation, up to 8–10 years when derated to I_sp about 8000 s.The goal of this paper is to show that current or near term NEP technology enables missions far beyond our immediate interplanetary backyard. In fact, by extending the semi-analytical approach used by Stuhlinger, with reasonable ratios α≡power/mass of the propulsion system (i.e., 0.1– 0.4 kW/kg), missions to the Kuiper Belt (40 AU and beyond) and even the so-called FOCAL mission (at 540 AU) become feasible with an attractive payload fraction and in times of order 10–15 years.Further results regarding missions to Sedna’s perihelion/aphelion, and to Oort’s cloud will also be presented, showing the constraints affecting their feasibility and mass budget.     

MOA2—an R&D paradigm buster enabling space propulsion by commercial applications

More than 60 years after the late Nobel laureate Hannes Alfvén had published a letter stating that oscillating magnetic fields can accelerate ionised matter via magneto–hydrodynamic interactions in a wave like fashion, the technical implementation of Alfvén waves for propulsive purposes has been proposed, patented and examined for the first time by a group of inventors.Consequently improved since then, the name of the latest concept, relying on magneto-acoustic waves to accelerate electric conductive matter, is MOA²—Magnetic field Oscillating Amplified Accelerator. Based on computer simulations, which were undertaken to get a first estimate on the performance of the system, MOA² is a corrosion free and highly flexible propulsion system, whose performance parameters might easily be adapted in operation, by changing the mass flow and/or the power level. As such the system is capable of delivering a maximum specific impulse of 13116 s (12.87 mN) at a power level of 11.16 kW, using Xe as propellant, but can also be attuned to provide a thrust of 236.5 mN (2411 s) at 6.15 kW of power. First tests—that are further described in this paper—have been conducted successfully with a 400 W prototype system at an ambient pressure of 0.20 Pa, delivered 9.24 mN of thrust at 1472 s I_SP, thereby underlining the feasibility of the concept.Based on these results, space propulsion is expected to be a prime application for MOA²—a claim that is supported by numerous applications such as Solar and/or Nuclear Electric Propulsion or even as an ‘afterburner system’ for Nuclear Thermal Propulsion. However, MOA² has so far seen most of its R&D impetus from terrestrial applications, like coating, semiconductor implantation and manufacturing as well as steel cutting. Based on this observation, MOA² resembles an R&D paradigm buster, as it is the first space propulsion system, whose R&D is driven primarily by its terrestrial applications. Different terrestrial applications exist, but the most successful scenarios so far revolve around MOA²'s unique features with respect to high throughput/low target temperature coatings on sensitive materials. In combination with its intrinsic high flexibility, MOA² is highly suited for a common space-terrestrial application research and utilisation strategy.This paper presents the recent developments of the MOA² R&D activities at Q₂ Technologie(s), the company in Vienna, Austria, which has been set up to further develop and test the magneto-acoustic wave technology and its applications.     

Development of an integrated countermeasure device for use in long-duration spaceflight

Prolonged weightlessness is associated with declines in musculoskeletal, cardiovascular, and sensorimotor health. Consequently, in-flight countermeasures are required to preserve astronaut health. We developed and tested a novel exercise countermeasure device (CCD) for use in spaceflight with the aim of preserving musculoskeletal and cardiovascular health along with an incorporated balance training component. Additionally, the CCD features a compact footprint, and a low power requirement. Methods: After design and development of the CCD, we carried out a training study to test its ability to improve cardiovascular and muscular fitness in healthy volunteers. Fourteen male and female subjects (41.4±9.0 years, 69.5±15.4 kg) completed 12 weeks (3 sessions per week) of concurrent strength and endurance training on the CCD. All training was conducted with the subject in orthostasis. When configured for spaceflight, subjects will be fixed to the device via a vest with loop attachments secured to subject load devices. Subjects were tested at baseline and after 12 weeks for 1-repetition max leg press strength (1RM), peak oxygen consumption (VO_2peak), and isokinetic joint torque (ISO) at the hip, knee, and ankle. Additionally, we evaluated subjects after 6 weeks of training for changes in VO_2peak and 1RM. Results: VO_2peak and 1RM improved after 6 weeks, with additional improvements after 12 weeks (1.95±0.5, 2.28±0.5, 2.47±0.6 L min^?1, and 131.2±63.9,182.8±75.0, 207.0±75.0 kg) for baseline, 6 weeks, and 12 weeks, respectively. ISO for hip adduction, adduction, and ankle plantar flexion improved after 12 weeks of training (70.3±39.5, 76.8±39.2, and 55.7±21.7 N m vs. 86.1±37.3, 85.1±34.3, and 62.1±26.4 N m, respectively). No changes were observed for ISO during hip flexion, knee extension, or knee flexion. Conclusions: The CCD is effective at improving cardiovascular fitness and isotonic leg strength in healthy adults. Further, the improvement in hip adductor and abductor torque provides support that the CCD may provide additional protection for the preservation of bone health at the hip.     

Measured surface temperatures of the Hayabusa capsule during re-entry determined from ground observation

The Hayabusa sample return capsule, which contained asteroid samples, re-entered the Earth's atmosphere on June 13, 2010. An ablative carbon-phenolic thermal protection system (TPS) was used to enable a safe return for the small capsule and the containing samples. Besides a research aircraft operated by NASA with a wide range of imaging and spectrographic cameras for remote sensing of the radiation of the Hayabusa capsule during its entry flight, observation from ground based stations has been realized. We participated in the ground based observation campaign with two instruments for spectroscopic and photometric measurements aiming to detect the surface temperature and the plasma radiation in front of the re-entering capsule. The system consists in an infrared camera and a wide range miniature fibre spectrometer. The paper presents the setup, the laboratory calibration procedure, and correction for transmission. The surface temperature of the capsule reached a peak of 3250 K when the capsule was at an altitude of 55.95 km. The thermographic camera measures independently slightly higher temperature at peak heating (3308 K).     

Thrust measurement of dimethyl ether arcjet thruster

The present paper describes thrust measurement results for an arcjet thruster using Dimethyl ether (DME) as the propellant. DME is an ether compound and can be stored as a liquid due to its relatively low freezing point and preferable vapor pressure. The thruster successfully produced high-voltage mode at DME mass flow rates above 30 mg/s, whereas it yielded low-voltage mode below 30 mg/s. Thrust measurements yielded a thrust of 0.15 N and a specific impulse of 270 s at a mass flow rate of 60 mg/s with a discharge power of 1300 W. The DME arcjet thruster was comparable to a conventional one for thrust and discharge power.     

Satellite orbit perturbations in a dusty Martian atmosphere

In this paper we calculate the effect of atmospheric dust on the orbital elements of a satellite. Dust storms that originate in the Martian surface may evolve into global storms in the atmosphere that can last for months can affect low orbiter and lander missions. We model the dust as a velocity-square depended drag force acting on a satellite and we derive an appropriate disturbing function that accounts for the effect of dust on the orbit, using a Lagrangean formulation. A first-order perturbation solution of Lagrange's planetary equations of motion indicates that for a local dust storm cloud that has a possible density of 8.323×10⁻¹⁰ kg m⁻³ at an altitude of 100 km affects the orbital semimajor axis of a 1000 kg satellite up −0.142 m day⁻¹. Regional dust storms of the same density may affect the semimajor axis up to of −0.418 m day⁻¹. Other orbital elements are also affected but to a lesser extent.     

Super-capacitor energy storage for micro-satellites: Feasibility and potential mission applications

Small satellites, weighting between 100 and 200 kg, have witnessed increasing use for a variety of space applications including remote sensing constellations and technology demonstrations. The energy storage/stored power demands of most spacecraft, including small satellites, are currently accommodated by rechargeable batteries—typically nickel–cadmium cells (specific energy of 50 Wh kg⁻¹), or more recently lithium-ion cells (150 Wh kg⁻¹). High energy density is a primary concern for spacecraft energy storage design, and these batteries have been sufficient for most applications. However, constraints on the allowable on-board battery size have limited peak power performance such that the maximum power supply capability of small satellites currently ranges between only 70 and 200 W. This relatively low maximum power limits the capabilities of small satellites in terms of payload design and selection. In order to enhance these satellites' power performance, the research reported in this paper focused on the implementation of super-capacitors as practical rechargeable energy storage medium, and as an alternative to chemical batteries. Compared to batteries, some super-capacitors are able to supply high power at high energy-efficiency, but unfortunately they still have a very low energy density (5–30 Wh kg⁻¹). However, the provision of this high power capability would considerably widen the range of small satellite applications.     

Prevention of muscle fibers atrophy during gravitational unloading: The effect of l-arginine administration

Gravitational unloading results in pronounced atrophy of m.soleus. Probably, the output of NO is controlled by the muscle activity. We hypothesized that NO may be involved in the protein metabolism and increase of its concentration in muscle can prevent atrophic changes induced by gravitational unloading. In order to test the hypothesis we applied NO donor l-arginine during gravitational unloading. 2.5-month-old male Wistar rats weighing 220–230g were divided into sedentary control group (CTR, n=7), 14-day hindlimb suspension (HS, n=7), 14 days of hindlimb suspension+l-arginine (HSL, n=7) (with a daily supplementation of 500 mg/kg wt l-arginine) and 14 days of hindlimb suspension+l-NAME (HSN, n=7) (90 mg/kg wt during 14 days). Cross sectional area (CSA) of slow twitch (ST) and fast twitch (FT) soleus muscle fibers decreased by 45% and 28% in the HS group (p<0.05) and 40% and 25% in the HSN group, as compared to the CTR group (p<0.05), respectively. CSA of ST and FT muscle fibers were 25% and 16% larger in the HSL group in comparison with the HS group (p<0.05), respectively. The atrophy of FT muscle fibers in the HSL group was completely prevented since FT fiber CSA had no significant differences from the CTR group. In HS group, the percentage of fibers revealing either gaps/disruption of the dystrophin layer of the myofiber surface membrane increased by 27% and 17%, respectively, as compared to the controls (CTR group, p<0.05). The destructions in dystrophin layer integrity and reductions of desmin content were significantly prevented in HSL group. NO concentration decreased by 60% in the HS group (as well as HSN group) and at the same time no changes were detectable in the HSL group. This fact indicates the compensation of NO content in the unloaded muscle under l-arginine administration. The levels of atrogin-1 mRNA were considerably altered in suspended animals (HS group: plus 27%, HSL group: minus 13%) as compared to the control level. Conclusion: l-arginine administration allows maintaining NO concentration in m.soleus at the level of cage control group, prevents from dystrophin layer destruction, decreases the atrogin mRNA concentration in the muscle and atrophy level under gravitational unloading.     

A harness for enhanced comfort and loading during treadmill exercise in space

IntroductionLocomotor and some resistance exercises in space require a gravity replacement force in order to allow 1g-like ground reaction forces to be generated. Currently bungee cords, or other loading devices, interface with the crew member through a harness with a waist belt and shoulder straps. Crew members often find the application of the required loads to be uncomfortable, particularly at the hips.MethodsAn experimental harness was built that differed from previous in-flight designs by having a wider, moldable waist belt and contoured shoulder straps with additional padding. Eight subjects ran at 100% body weight (BW) loading for a total duration of 30 min per day on 12 days over a 3-week period in simulated 0-g conditions using horizontal suspension. A 100 mm Visual Analog Scale (VAS)¹ was used to assess harness-related and lower extremity discomfort at the end of each run.ResultsThe overall rating of harness discomfort decreased from 27 mm on the 100 mm scale on day 1 to 10 mm on day 12, with significant decreases recorded for the back and hip regions as well as the overall harness.DiscussionThe experimental harness allows for repeated exposure to 30-minute bouts of 100% BW loaded simulated 0-g running with levels of discomfort less than 30 mm on a VAS scale of 0–100 mm. We believe that the use of such a harness during on-orbit exercise countermeasures may allow exercise to be performed at levels which are more effective in preventing bone and muscle loss.     

Very high delta-V missions to the edge of the solar system and beyond enabled by the dual-stage 4-grid ion thruster concept

A new and innovative type of gridded ion thruster, the “Dual-Stage 4-Grid” or DS4G concept, has been proposed and its predicted high performance validated under an ESA research, development and test programme. The DS4G concept is able to operate at very high specific impulse and thrust density values well in excess of conventional 3-grid ion thrusters at the expense of a higher power-to-thrust ratio. This makes it a possible candidate for ambitious missions requiring very high delta-V capability and high power. Such missions include 100 kW-level multi-ton probes based on nuclear and solar electric propulsion (SEP) to distant Kuiper Belt Object and inner Oort cloud objects, and to the Local Interstellar medium. In this paper, the DS4G concept is introduced and its application to this mission class is investigated. Benefits of using the DS4G over conventional thrusters include reduced transfer time and increased payload mass, if suitably advanced lightweight power system technologies are developed.A mission-level optimisation is performed (launch, spacecraft system design and low-thrust trajectory combined) in order to find design solutions with minimum transfer time, maximum scientific payload mass, and to explore the influence of power system specific mass. It is found that the DS4G enables an 8-ton spacecraft with a payload mass of 400 kg, equipped with a 65 kW nuclear reactor with specific mass 25 kg/kW (e.g. Topaz-type with Brayton cycle conversion) to reach 200 AU in 23 years after an Earth escape launch by Ariane 5. In this scenario, the optimum specific impulse for the mission is over 10,000 s, which is well within the capabilities of a single 65 kW DS4G thruster. It is also found that an interstellar probe mission to 200 AU could be accomplished in 25 years using a “medium-term” SEP system with a lightweight 155 kW solar array (2 kg/kW specific mass) and thruster PPU (3.7 kg/kW) and an Earth escape launch on Ariane 5. In this case, the optimum specific impulse is lower at 3500 s which is well within conventional gridded ion thruster capability.     

Impaired T-wave amplitude adaptation to heart-rate induced by cardiac deconditioning after 5-days of head-down bed-rest

The study of QT/RR relationship is important for the clinical evaluation of possible risk of acquired or congenital ventricular tachyarrhythmias. In the hypothesis that microgravity exposure could induce changes in the repolarization mechanisms, our aim was to test if a short 5-days strict 6° head-down bed-rest (HDBR) could induce alterations in the QT/RR relationship and spatial repolarization heterogeneity. Twenty-two healthy men (mean age 31±6) were enrolled as part of the European Space Agency HDBR studies. High fidelity (1000 Hz) 24 h Holter ECG (12-leads, Mortara Instrument) was acquired before (PRE), the last day of HDBR (HDT5), and four days after its conclusion (POST). The night period (23:00–06:30) was selected for analysis. X, Y, Z leads were derived and the vectorcardiogram computed. Selective beat averaging was used to obtain averages of P–QRS–T complexes preceded by the same RR (10 ms bin amplitude, in the range 900–1200 ms). For each averaged waveform (i.e., one for each bin), T-wave maximum amplitude (Tmax), T-wave area (Tarea), RTapex, RTend, ventricular gradient (VG) magnitude and spatial QRS-T angle were computed. Non-parametric Friedman test was applied. Compared to PRE, at HDT5 both RTapex and RTend resulted shortened (−4%), with a decrease in T-wave amplitude (−8%) and area (−13%). VG was diminished by 10%, and QRS-T angle increased by 14°. At POST, QT duration and area parameters, as well as QRS-T angle were restored while Tmax resulted larger than PRE (+5%) and VG was still decreased by 3%. Also, a marked loss in strength of the linear regression with RR was found at HDT5 in Tmax and Tarea, that could represent a new dynamic marker of increased risk for life-threatening arrhythmias. Despite the short-term HDBR, ventricular repolarization during the night period was affected. This should be taken into account in astronauts for risk assessment during space flight.